The proposed research consists of four related but semi-independent projects. They are: 1) The identity of the stimulating cell or cells in the Mixed Lymphocyte Culture (MLC) and their requirement in other thymus dependent responses. 2) The determinants of the thymus- macrophage recognition system. 3) The cellular dynamics of the thymus- dependent synthesis of immunoglobulin. 4) Suppression in graft-versus- host reactions. The central phenomenon of this investigation is the MLC, a proliferative response of thymus derived cells. Although the MLC is present in all species with a thymus which have been studied, a function for this universal phenomenon is not demonstrated. We have proposed that the MLC represents a self-recognition system the function of which is to respond to exogenous antigens indirectly through the specific alterations they produce in the self-patterns of the surface membrane of a specialized stimulating cell.